Adhesive coating compositions are used on a large scale, especially for coating sheet-like materials in order to decrease the tendency of adhesive products to adhere to these surfaces. Adhesive coating compositions are used, for example, to coat papers or films, which are to serve as supports for pressure-sensitive labels. The labels, provided with a pressure-sensitive adhesive, adhere to the coated surface to a still sufficient extent, to make it possible to handle the support films with the pressure-sensitive labels. It must, however, be possible to pull the labels from the coated support film without significantly affecting their adhesiveness for later use. Other possible applications for adhesive coating compositions are wrapping papers, especially for packaging adhesive goods. Such adhesive papers or films are used, for example, to package foods or technical products such as bitumen.
Such organopolysiloxanes, modified with (meth)acrylate ester groups, are also used to coat printed circuit boards equipped with electronic components. For this application, they serve especially as protection against mechanical effects and corrosive gases or vapors.
The German Patent No. 2,948,708 discloses a method for producing organopolysiloxanes, which are modified with pentaerythritol triacylate or pentaerythritol trimethacrylate esters, from organochloropolysiloxanes, optionally with addition of HCl-binding neutralizing agents. In this patent, organopolysiloxanes of the formula ##STR1## (R.sup.1 is alkyl with 1 to 4 carbon atoms, vinyl and/or phenyl, with the proviso that at least 90 mole percent of the R.sup.1 groups are methyl; a is 1.8 to 2.2, b is 0.004 to 0.5) are first reacted with, based on the SiCl groups, at least 2 molar amounts of a dialkylamine, the alkyl groups of which in each case have 3 to 5 carbon atoms, the carbon atoms adjacent to the nitrogen having at most one hydrogen atom each. The reaction product is allowed to react with at least equimolar amounts of pentaerythritol triacrylate or pentaerythritol methacrylate and the end product is then separated from solid components suspended in it by known means.
A coating composition prepared by this method exhibits good adhesive properties. Adhesive tapes in contact with the coating composition largely retain their adhesiveness towards untreated substrates. It has, however, been ascertained that the property of adhesiveness must always be considered and evaluated in connection with the chemical constitution and structure of the adhesive, towards which the coating agent is to show adhesive properties. The adhesive coating material disclosed in the German Patent No. 2,948,708 therefore does not provide satisfactory results in all cases, since its properties cannot be adapted to the different adhesives.
Improved properties are shown by (meth)acrylate ester-modified organopolysiloxane mixtures, which are characterized in that they consist of an equilibrated organopolysiloxane with, on the average, &gt;25 to &lt;200 silicon atoms and 2 to 30% by weight of organopolysiloxanes with, on the average, 2 to 25 silicon atoms and 2 to 30% by weight organopolysiloxanes with, on the average, 200 to 2,000 silicon atoms. The organopolysiloxanes, contained in this ternary mixture, have different tasks. The low molecular weight fraction essentially has the task of assuring that the coating composition adheres to the substrate. The high molecular weight fraction, on the other hand, serves primarily the purpose to attain the desired adhesiveness of the coating composition. The middle fraction is the curable matrix, which is responsible especially for the physical properties of the coating composition. Those skilled in the art will understand that this is only a simplified description of the properties and tasks of the three different fractions, since the complex properties, which an adhesive coating material must have, can only be obtained by the united efforts of the three components. With the modified organopolysiloxane mixture of the German Patent No. 3,426,087, it has become possible to improve, on the one hand, the adhesive properties of the mixture towards adhesive surface and, on the other, the adhesive properties towards the substrate, on which the coating material is applied and on which it is cured. However, even with this coating mass it has turned out that the properties cannot be adapted adequately to the different adhesives.
In the European Offenlegungsschrift No. 0 159 683, electron beam-curable liquid coating materials are described, which should contain:
1. 60 to 95 parts of an organopolysiloxane with more than about 25 siloxane groups per molecule and 2 to 10 parts of reacted carbinol groups per molecule, the rest of the substituents on the silicon being hydrocarbon groups with 1 to 20 carbon atoms; the term reacted carbinol groups is understood to mean esters of acrylic acid, methacrylic acid or mixtures or ethers of a hydroxyalkyl ester of these acids, the alkyl group containing 2 to 4 carbon atoms; essentially, unreacted carbinol groups should no longer be present, so that the hydroxyl number is &lt;10.
2. 3 to 25 parts of a polyester of a multihydric alcohol with acrylic acid, methacrylic acid or mixtures thereof, the multihydric alcohol having 2 to 4 hydroxyl groups per molecule and a molecular weight of &lt;1,200;
3. 1 to 10 parts of acrylic acid, methacrylic acid or mixtures of these acids.
The additional use of the (meth)acrylate ester of a polyalcohol increases the curing rate, but has a disadvantageous effect on the flexibility and adhesiveness of the coating material as a result of the increase in the organic portion. The content of free acrylic or methacrylic acid is an additional disadvantage, which leads to an annoying odor and makes the processing difficult during the application on the material to be coated.
Because they can be cured by radiation, organopolysiloxanes with acrylate ester groups have been described for a series of other possible applications. Organopolysiloxanes, modified with acrylate ester groups, are thus used as coating lacquers that are to be poured into and around electrical and electronic components and also for the manufacture of molded objects. The following Offenlegungsschriften, Auslegeschriften and patents are cited with regard to the possible structures of such acrylate ester group-modified polysiloxanes:
The German Auslegeschrift No. 2,335,118 relates to optionally substituted acrylate group-containing organopolysiloxanes of the general formula ##STR2## (R=hydrogen or univalent hydrocarbon groups with 1 to 12 carbon atoms; R'=univalent, optionally halogenated hydrocarbon groups or cyanoalkyl groups with 1 to 8 carbon atoms; R"=divalent hydrocarbon groups with 1 to 18 carbon atoms or C--O--C bonds-containing divalent hydrocarbon groups; R'"=R""O.sub.0.5 or R'.sub.3 SiO.sub.0.5 ; Z=OR"", R"" or OSiR'.sub.3 ; R""=alkyl group with 1 to 12 carbon atoms; a and b each represent numbers from 1 to 20,000; c is a number from 0 to 3; e is a number from 0 to 2; at least one of the Z groups is OR"" when c=0). The siloxane polymers can be used as intermediates in the synthesis of copolymers, which contain organopolysiloxane segments and find use as coating compositions. Moreover, these acrylate-functional siloxane polymers can serve as sizes and protective coating compositions for paper and fabric. However, these products are unsuitable for the preparation of adhesive coating materials. Furthermore, the linear, diacrylate-modified polysiloxanes of the German Auslegeschrift No. 2,335,118 by definition have alkoxy groups, which can be split off hydrolytically and lead to further cross linking of the polysiloxanes with a deterioration of the elastic properties, which are important for a coating composition.
The German Offenlegungsschrift No. 3,044,237 discloses polysiloxanes with lateral acrylate ester groups, which can be synthesized by the reaction of epoxy-functional siloxanes of a particular structure with acrylic acid. The products obtained are curable by radiation. They can be used as low-viscosity lacquers for application via conventional oil-based printing inks. As adhesive coatings compositions, the products can be used only with considerable limitations, since there is a hydroxyl group for each acrylate ester group.
In the U.S. Pat. No. 4,568,566, curable silicone preparations are described, which comprise:
(a) 75 to 100 mole percent of chemically bound siloxy units of the formula R.sub.3 SiO.sub.0.5, RSiO.sub.1.5 and SiO.sub.2, as well as
(b) 0 to 25 mole percent of R.sub.2 SiO units, a number of the R units having the formula ##STR3## wherein R.sup.1 is a hydrogen group or a hydrocarbon group with 1 to 12 carbon atoms and R.sup.2 is a divalent hydrocarbon group or an oxyalkylene group. These curable preparations are used especially for coating electronic components and as a coating material for optical fibers. Because the content of R.sub.2 SiO units is too low, they are not suitable as adhesive coating materials for sheet-like supports.
The object of the European Offenlegungsschrift No. 0 152 179 is a silicone preparation, which can be cured into an elastomer. This preparation comprises (a) a silicone resin with linear structure and, on the average, at least 150 siloxane units, as well as terminally linked acrylic acid groups, the region in between the terminal groups being free of acrylic acid groups; (b) at least 10% of finely divided silica and (c) a photoinitiator. These materials are to be used as adhesives and as casting compositions.
Finally, reference is made to the European Offenlegungsschrift No. 0 169 592, which relates to an optical glass fiber with a synthetic resin covering, with a glass fiber and an enveloping layer of artificial rubber with a refractive index, which is higher than that of the outer layer of the glass fiber, the artificial rubber being formed from a curable synthetic resin composition, which has a copolymer that contains, as monomeric units, dimethylsiloxane and at least one siloxane from the group comprising methylphenylsiloxane and diphenylsiloxane. The siloxane copolymer contains at least two acrylate ester groups per molecule. As a distinguishing feature, the curable synthetic resin composition additionally contains a polyurethane acrylate with an average molecular weight of 3,000. The polysiloxane named in the claim may have the following formula: ##STR4## It is an essential conditions that these polysiloxanes have phenyl groups linked to silicon. The phenyl group content is necessary in order to approximate the refractive index of the coating composition to that of the glass optical fiber. From the use of these siloxanes in combination with a polyurethane acrylate for coating optical glass fibers, it cannot be concluded that such compounds can possibly also be used as adhesive compositions.
In the journal "Makromolekulare Chemie" (Macromolecular Chemistry, Rapid Communication), 7, (1986), pages 703 to 707, the synthesis of linear methylpolysiloxanes with terminal methacrylate ester groups is described. For this reaction, an allyl epoxypropyl ether first of all undergoes an addition reaction with .alpha.,.omega.-hydrogendimethylpolysiloxane in the presence of chloroplatinic acid. The diepoxide formed is subsequently reacted with methacrylic acid in the presence of chromium diisopropyl salicylate to form the desired methacrylate esters. These esters may be present in two isomeric forms: ##STR5##
If the (meth)acrylate esters are prepared by methods of the state of the art starting out with epoxy-functional siloxanes, the epoxy groups are reacted with (meth)acrylic acid. In this reaction, (meth)acrylate monoesters with a vicinal hydroxyl group are formed by opening the epoxide ring.